1. Field of the Invention
The present invention relates to a permselective membrane type reactor. More particularly, it relates to a permselective membrane type reactor which simultaneously performs suppression of a methanation reaction and proceeding of a shift reaction while suppressing deterioration of a thinned permselective membrane, whereby hydrogen can efficiently be recovered.
2. Description of the Related Art
A hydrogen gas is used as a basic material gas for petrochemistry in large quantities, and largely expected as a clean energy source. The hydrogen gas which is used for such a purpose is formed from a main material gas including a hydrocarbon such as methane, butane, or kerosene, or an oxygen-containing hydrocarbon such as methanol by the utilization of a reforming reaction, a partial oxidizing reaction, or a decomposition reaction, and the resultant byproducts of carbon monoxide and water are then subjected to a shift reaction. Hydrogen formed in this manner can be separated and obtained from the gas by use of a permselective membrane such as a palladium alloy membrane which allows hydrogen to selectively permeate therethrough.
As described above, the shift reaction is performed after the reforming reaction or the like in a hydrogen production process. From viewpoints of a thermodynamic restriction and a kinetic problem, the water-gas shift reaction is usually constituted of two steps of a high temperature water-gas shift reaction and a low temperature water-gas shift reaction. Industrially, in the high temperature water-gas shift reaction at 300 to 500° C., an iron oxide-chromium oxide type composite catalyst (hereinafter merely referred to as an iron-chromium catalyst) is usually used. A water-gas shift reaction using a noble metal-based catalyst is also investigated (for example, see Patent Document 1).
The water-gas shift reaction is represented by the following formula (a):CO+H2O═CO2+H2  (a).
In the shift reaction in which a reforming gas is used as the material gas, the following methanation reaction could occur as a side reaction. However, when the iron chromium catalyst is used, the only water-gas shift reaction selectively proceeds.
The methanation reaction is represented by the following formula (b):CO+3H2═CH4+H2O  (b).
Moreover, a membrane type reactor (a permselective membrane type reactor) which simultaneously performs the above water-gas shift reaction and separation of hydrogen is also known. As a use example of the membrane type reactor, for example, a membrane type reactor is prepared using a Pd membrane having a membrane thickness of 20 μm and an iron-chromium catalyst, and a principle of an effect of the reactor with respect to the water-gas shift reaction is demonstrated (for example, see Non-Patent Document 1).
In the permselective membrane type reactor, a product is selectively removed from a reversible reaction system to provide an advantage that the reaction apparently proceeds in excess of an extent of equilibrium reaction. Such a permselective membrane type reactor has a merit that the reactor can be a compact device because the reaction and the separation of hydrogen can simultaneously be performed as described above. In addition, the hydrogen gas is extracted to shift equilibrium of the reaction toward a forming side, whereby a reaction temperature can be lowered. In consequence, effects such as lowering of an operation temperature, suppression of deterioration of a metal member and energy saving can be expected.
[Patent Document 1] JP-A-2004-284912
[Non-Patent Document 1] Eiichi Kikuchi et al., Chemistry Letters (1989) 489-492
In such a conventional permselective membrane type reactor, a Pd membrane thickness is large, so that a permeation performance of a Pd membrane is not sufficient, and it has been difficult to efficiently recover hydrogen. Reduction of the Pd membrane thickness not only improves the permeation performance but also leads to reduction of an amount of Pd to be used. Therefore, the reduction of the membrane thickness is preferable from the viewpoint of cost.
When an iron-based catalyst is used, only a water-gas shift reaction at 300 to 500° C. selectively proceeds, and a methanation hardly occurs. As described above, to improve the permeation performance of the Pd membrane, it is preferable to reduce the thickness of the Pd membrane. However, in the permselective membrane type reactor in which a thin Pd membrane and the conventional iron-based catalyst are used, in a case where the Pd membrane comes in contact with iron as a catalyst component at a high temperature, there is a problem that the permselective membrane deteriorates owing to the reaction in a remarkably short time. The thinner the Pd membrane is, the more remarkable a deterioration rate of the permselective membrane becomes. When the reaction temperature rises, the deterioration rate increases more remarkably.
On the other hand, in recent years, as a catalyst which replaces the conventional iron-based catalyst or a copper-based catalyst, a noble metal-based shift catalyst having a high resistance to oxidation and a high activity has been reported. However, when the noble metal-based catalyst is used, there is a problem that the methanation proceeds to consume a target product, hydrogen. Also, the noble metal-based catalyst is expensive and has a problem in the cost aspect. In addition, the noble metal-based catalyst has slightly poor selectivity as compared with the iron-based catalyst. Therefore, as a result, there has been a problem that the performance of the permselective membrane type reactor deteriorates.